Variable wave guide attenuator



VARIABLE WAVE GUIDE ATTENUATOR Filed Feb. 21, 1952 /NVENTOR Henry J R//ef United States a Patent VARIABLE WAVE GUIDE ATTENUATOR Henry J. Riblet, Wellesley, Mass. Application February 21, 1952, Serial No. 272,813

3 Claims. (Cl. 333-.81-)

This invention relates generally to waveguide attenuators and in particular to a novel variable waveguide attenuator which is capable of absorbing high power, has little or no attenuation at minimum setting and is inherently Well matched.

In radar and communication systems there is frequently required some method for changing the radio frequency power level. At low and medium power levels this may be accomplished by moving a fiat sheet of lossy material laterally across the waveguide as is described in Technique of Microwave Measurements, Radiation Laboratory Series, pp. 774799. These devices, although very satisfactory for many applications, suffer from limitations on power absorption, minimum insertion loss and V. S. W. R. which are inherent in their design.

The object of this invention is to provide a variable waveguide attenuator which will overcome these objections.

Other and further objects and features of the invention will become apparent from the following description of an illustrative embodiment thereof, reference being made to the accompanying drawings wherein:

Figure 1 is an isometric view of a variable waveguide attenuator with certain parts cut away to show interior structure.

Figure 2 is a partial view of the device showing the lossy absorbing material located in a position for high attenuation.

Figure 3 is a schematic drawing which will be useful in explaining the operation of the device.

Referring now to Figures 1 and 2, two substantially identical rectangular waveguides 1 and 3 are disposed parallel to each other, each with a wide wall adjacent and conductively joined to that of the other. The two waveguides are directionally coupled together by means of a hybrid junction A which is shown for convenience as a two slot top wall hybrid junction discussed in copending application, Serial No. 118,175, filed September 27, 1949. This consists of a pair of properly dimensioned slots 4 cut in the common wall 5 between the two waveguides 1 and 3. The waveguides 1 and 3 have input and output terminals I-2 and 0-3 respectively. A centrally located metal plate 6 extends between the middle of the top waveguide 3 and the middle of the bottom waveguide 1. This plate 6 is on the opposite side of the hybrid junction A from the terminal I-2 and 0-3. Four similar plugs of lossy material 7 are mounted and held together by a common segment connection 8 and so spaced that clearance is provided for the common wall 5 and the metal plate 6. While the invention herein is shown with a rectangular waveguide, it also applies to other types of guides where a common wall or abutting walls are used.

The operation of this variable attenuator then proceeds as follows and is explained with the help of Figure 3. The incident voltage enters at terminal I-2 and divides equally at the hybrid junction A where half of the energy proceeds to the right in waveguide 3 and half proceeds to the right in waveguide 1. When this energy encounters ice the metal plate. 6 it is reflected in its entirety since the length of the plate 6 has been chosen so that no energy propagates through the waveguide sections beyond the cut-off. The phase of the reflected voltages is such that this energy recombines in the hybrid junction A and leaves the circuit at terminalO-S. It is clear that under these 1 conditions ,of operation, the losses in the circuit will be very small since there are no lossy elements in the circuit.

In order to obtain attenuation, .it is necessary to slide the lossy plugs 7 in such a manner that they project beyond the metal plate 6. It is clear that energy incident upon them will be partially reflected and partially absorbed before reaching the metal plate 6. It is important that the metal plate 6 be long enough so that the sections of waveguide 9 which it forms are beyond cut-off even when filled with the lossy plugs 7. How the total attenuation will increase with increasing penetration of the lossy plugs 7 will be clear to anyone familiar with the art.

Many modifications of the invention will occur to those skilled in the art. For example, various other types of directional coupler like, hybrid junctions may be used in the invention at the option of the user. The two waveguides 1 and 3 may be arranged in configurations different from that illustrated. Various other arrangements are possible, such as flat arrangement, wherein the waveguides are coupled through the narrow wall. It is, therefore, intended that the claims that follow shall not be limited to the particular details of the embodiment of the invention herein illustrated, but shall be limited only by the prior art.

The lossy plugs 7 may be composed of a mixture of plastic and powdered metal molded as a unit such as polystyrene and iron particles, or powdered metal and powdered carbon mixed and molded or such trade named products as Aquadag or Polyiron.

The plate 6 may be a wave length long corresponding to a frequency within the band transmitted through the attenuator or it may be longer than this if desired. The plate 6 is positioned between the hybrid junction and the normal unused positions of the lossy plug 7, but the plate 6 must be sufficiently behind the hybrid junction to allow the lossy plug to extend in front of the plate to attenuate the waves transmitted by the hybrid junction. A distance of at least about where A is a wave length within the transmission band should be allowed for this purpose.

Having now described my invention, I claim:

1. A variable waveguide attenuator operative in a predetermined radio frequency transmission band comprising, first and second rectangular waveguide sections each having parallel broad and narrow walls and a common broad wall, a symmetrical directional coupler-like hybrid junction formed in said common broad wall and thereby interconnecting said first and second waveguide sections, means for applying energy to be attenuated to an input end of said first waveguide section, means for deriving attenuated energy from an output end of said second waveguide section, said input and output ends of said waveguide sections being both disposed to a common side of said hybrid junction, a planar conductive reflecting plate extending through said first and second waveguide sections between said broad walls, said plate being perpendicular to said common broad wall and disposed centrally between and parallel to the narrow walls of each of said waveguide sections on the side of said hybrid junction opposite said input and output ends, said plate thereby defining four cut-off waveguide sections for the frequencies of said transmission band, and four similar lossy plug elements arranged for displacement in unison movable from a maximum attenuating penetrating posi-,

tion whereineach of said plugs projects respectively into one of said cut-off waveguide sections beyond said metal plate and axially toward said hybrid junction to a. substantially unattenuating withdrawn position wherein said plugs are displaced axially from said plate and said four cut-off waveguide sections in the direction toward the ends of insertion into said waveguide sections.

2. A variable waveguide attenuator operative in a predetermined radio frequency transmission band in accordance with claim 1 wherein the ends of said four lossy plugs directed toward said hybrid junction are uniformly tapered.

3. A variable waveguide attenuator operative in a predetermined radio frequency transmission band in accordance with claim 1 wherein the edge. of said planar plate nearersaid hybrid junction is displaced therefrom along the longitudinal axes of said waveguide sections toward said ends through which said plugs are inserted more than one quarter wavelength measured at a frequency within said transmission band.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,123 King Apr. 16, 1940 2,423,396 Linder July 1, 1947 2,487,547 Harvey Nov. 8, 1949 2,579,327 Lund Dec. 18, 1951 2,580,678 Hansen Jan. 1, 1952 2,594,874 Cohn Apr. 29, 1952 2,632,809 Riblet Mar. 24, 1953 2,634,331 Honda Apr. 7, 1953 2,682,641 Sensiper June 29, 1954 2,709,241 Riblet May 24, 1955 2,709,789 Worrell May 31, 1955 FOREIGN PATENTS 446,640 Italy Mar. 23, 1949 

